Thermoplastic polyester resins, such as polyalkylene terephthalate resins (e.g., polybutylene terephthalate) are well known engineering resins that have been used extensively in various fields owing to their excellent mechanical and electrical properties, as well as their physical and chemical characteristics. Thus, thermoplastic polyester resins have been used to form molded parts and components for automotive, electrical and electronic end-use applications.
In those end-use applications requiring flame-retardant properties, such as parts and/or components for electrical and electronic apparatus, organohalogen primary flame retardants and one or more auxiliary flame retardants have typically been blended with thermoplastic polyesters in order to impart the desired level of flame-retardant properties thereto. Furthermore, improved flame-retardant properties have been imparted to thermoplastic polyester resins by introducing a flame-retardant group (usually a halogen-containing group) into the polyester resin during polymerization, and then augmenting the flame-retardant property enhancement that is achieved by blending an auxiliary flame-retardant with the polyester resin.
The costs of organohalogen flame retardants is relatively expensive as compared to the thermoplastic polyester base resin with which it is blended. As a consequence, those in the art have typically employed relatively inexpensive antimony oxide as an auxiliary flame-retardant in as large amounts as possible in order to reduce the amounts (and hence costs) of the primary organohalogen flame-retardant.
In this connection, antimony trioxide is representative of the class of antimony oxides that may be blended with thermoplastic polyesters as an auxiliary flame-retardant. However, antimony trioxide is typically in the form of a fine powder having a mean particle diameter of 0.1 to several microns. As a result, when directly blended with polyester base resins and/or a primary flame retardant, the antimony trioxide is likely to agglomerate and accumulate upon various parts of the extrusion processing equipment (e.g., the extruder screw, pipe walls, and the like). Thus, periodic equipment down time is needed in order to remove the accumulated antimony trioxide thereby resulting in lost production and increased maintenance costs.
One technique that has been employed in the art in order to avoid the above-noted problem is to use a so-called "two-stage" blending technique when antimony trioxide is added to polyester base resins. According to this technique, the antimony trioxide is preblended with powdered (pulverized) polyester base resin and/or a primary flame retardant to form a powdery master batch. This master batch may then be melt-blended with feed pellets of the thermoplastic polyester resin. Apparently, the powdery master batch serves as a carrier of sorts to more evenly disperse the antimony trioxide throughout the resulting thermoplastic polyester base resin and thereby minimize agglomeration. However, the down side of this two-stage blending process is that it is more labor-intensive as compared to direct blending techniques so that production costs are higher. Moreover, even when such a two-stage blending process is followed, a homogenous distribution of the antimony trioxide throughout the thermoplastic polyester base resin may still not result. Thus, molded polyester parts of of less than desired quality results.
What has been needed therefore is a thermoplastic polyester molding resin that has adequate flame-retardant properties, but yet allows meaningful amounts of antimony oxides to be incorporated thereinto as an auxiliary flame-retardant without the attendant problems noted above. It is towards fulfilling such a need that the present invention is directed. Broadly, the present invention is embodied in moldable thermoplastic polyester resins and to molded articles formed of the same which include a thermoplastic polyester base resin, and a flame-retardant effective amount of (i) an organohalogen flame-retardant, and (ii) an auxiliary flame retardant in the form of antimony oxide having a sulfate content of 0.05% by weight or less.
Preferably, the flame-retardant package of the organohalogen flame retardant and the antimony oxide flame retardant are present between 1 to 40 parts by weight and between 1 to 30 parts by weight, respectively, based on 100 parts by weight of the thermoplastic polyester base resin. The compositions of this invention may optionally contain an effective amount of between 0.01 to 5 parts by weight (based on 100 parts by weight of the thermoplastic polyester base resin) of a polytetrafluoroethylene resin to enable the compositions of this invention to pass the burn test (V-O) of Underwriters Laboratories Standard UL-94.
Further advantages and aspects of this invention will become more clear after careful consideration is given to the following detailed description of the preferred exemplary embodiments.